Systems and methods for computing a relative path delay between multiple transmission sources

ABSTRACT

Systems and methods are provided for computing a relative path delay between multiple transmitting source to select a source that is closest to a receiving device. Preamble sequences unique to each source are received by a receiving device. The receiving devices determines based on a channel estimation or differential algorithm which transmitting source is closer to the receiving device. The channel estimation algorithm computes the path delay based on a channel estimation correlation at different preamble sequence indices. The differential algorithm computes the path delay based on a correlation between the received and transmitted preamble sequences at different preamble sequence indices. The receiving device selects the closer of the multiple sources to be the source from which to extract data.

CROSS REFERENCE TO A RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/954,669, filed Aug. 8, 2007, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to systems and methods for computing arelative path delay between multiple transmission sources, and moreparticularly to selecting a source which is closest to the receivingdevice based on a relative path delay estimation in orthogonal frequencydivision multiplexing (OFDM) systems.

Typically, a receiver (i.e., a mobile station (MS)) in a multipletransmission source (i.e., multiple base stations (BS)) system receivesthe same data signal from each transmission source and has to select oneof the transmission sources to be the serving transmission source (i.e.,the source from which the data is extracted and utilized). The receivermeasures the power of each received signal (e.g., using a measure ofReceived Signal Strength Indicator (RSSI)) and selects the transmissionsource of the signal having the largest amount of power to be theserving transmission source.

However, because power of a signal received at a device does notcorrelate well with the distance to the transmission source from whichthe signal originated, these systems lack the capability to select thetransmission source that is closest to the receiver. Additionally,although the transmission sources are synchronized to transmit thesignals at the same time, the time at which the signals are receiveddepends on the path delay between the receiver and the transmissionsources. Thus, because these systems do not take into account distancewhen selecting a particular serving transmission source, the closesttransmission source is not always selected which negatively impactsperformance.

Accordingly, it is desirable to provide enhanced systems and methods forselecting a closest serving transmission source among multipletransmission sources to improve performance.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, systems andmethods are provided for computing a relative path delay for multipletransmission sources, and more specifically for computing distances andpath delays between the transmission sources in order to improve overallperformance of OFDM systems.

Signals from multiple base stations (e.g., multiple sources) may bereceived by a mobile station (e.g., a receiving device). The mobilestation may need to select the closest signal as its source of data inorder to improve performance. Each source is identified with a uniquepreamble sequence that is known to the receiving device (e.g., preamblesequence defined by the WiMax standard).

Based on the preamble sequence that is received at the receiving device,an algorithm may be performed to determine the distance or path delay tothe transmitting source. In some embodiments, the receiving devicecomputes the path delay of each one of the sources based on a channelestimation algorithm. The channel estimation algorithm correlates thechannel estimate associated with a preamble sequence of a base stationwith the channel estimate evaluated at a shifted index value of thepreamble sequence. The path delays are then compared to determine whichone of the sources is closer to the receiving device.

In some embodiments, the receiving device computes the path delay ofeach one of the sources based on a differential algorithm. The receivedpreamble sequence is correlated with the received preamble sequenceevaluated at a shifted index value of the received preamble sequence.Similarly, the transmitted preamble sequence is correlated with thetransmitted preamble sequence evaluated at a shifted index value of thetransmitted preamble sequence. The transmitted preamble sequencecorrelation and the received preamble sequence correlation are thencorrelated with each other to determine the path delay. In particular,the magnitude of the transmitted and received preamble sequencecorrelation provides the RSSI of the signal associated with a particularsource and the phase of the transmitted and received preamble sequencecorrelation provides the path delay to the particular source. The pathdelays are then compared to determine which one of the sources is closerto the receiving device.

The signal from the closer of the multiple sources is selected to be theserving source (e.g., the source from which data is extracted). Forexample, the receiving device may switch from one source to another ifit determines (based on the channel estimation algorithm or differentialalgorithm) that one source is closer to the receiving device than theother.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention, its nature andvarious advantages will be more apparent upon consideration of thefollowing detailed description, taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a diagram of an illustrative relative path delay measurementsystem in accordance with an embodiment of the present invention;

FIG. 2 is a diagram of an illustrative relative path delay estimationcircuit in accordance with an embodiment of the present invention;

FIGS. 3 and 4 are more detailed diagrams of a relative path delayestimation circuits in accordance with an embodiment of the presentinvention;

FIG. 5 illustrates a process for computing relative path delay betweenmultiple sources in accordance with an embodiment of the presentinvention;

FIG. 6A is a block diagram of an exemplary vehicle that can employ thedisclosed technology;

FIG. 6B is a block diagram of an exemplary cell phone that can employthe disclosed technology; and

FIG. 6C is a block diagram of an exemplary media player that can employthe disclosed technology.

DETAILED DESCRIPTION

This invention generally relates to relative path delay measurement fora system having multiple transmission sources. For illustrativepurposes, this invention will be described in the context of a WiMaxsystem (i.e., IEEE 802.16 standard) but it should be understood that theteachings of the invention can be applied to any system having multipletransmission sources, such as WiFi, cellular systems, GPS systems, andthe like.

FIG. 1 is a diagram of an illustrative relative path delay measurementsystem 100 in accordance with an embodiment of the present invention.System 100 includes transmission sources 120 and 130 (which may bereferred to below as base stations (BS) 120 and 130) and a receivingdevice 110 (which may be referred to below as mobile station (MS) 110).

Each base station 120 and 130 communicates with mobile station 110through a respective transmission link 122 and 132. Transmission link122 or 132 may be any wireless communication path such as WiFi, WiMax,BlueTooth, or the like. Mobile station 110 receives the signalstransmitted from each base station through one or more antennas 112. Insome implementations, the base stations 120 and 130 transmit the datasignals in a multiple input multiple output (MIMO) fashion. In suchcircumstances, mobile station 110 may have multiple antennas 112 forreceiving signals from each of base stations 120 and 130. This will bediscussed in more detail below.

Base station 120 may be at a distance d₁ away from mobile station 110.Base station 130 may be at a farther distance d₂ away from mobilestation 110 than base station 120. Mobile station 110 may initiallycommunicate with base station 130 and as such the base station isreferred to as the serving base station (or anchor base station). Whenmobile station 110 encounters a signal from a second base station (e.g.,base station 120), mobile station 110 may compute a relative path delaybetween the two base stations to determine which base station 120 or 130is closer.

For example, mobile station 110 may assign a relative value of ‘0’ tothe serving base station (e.g., base station 130). Mobile station 110may then determine the relative path delay by computing the differencebetween the distances d₁ and d₂ to each base station and divide thatdifference by c (i.e., the speed of light) and assign the relative pathdelay to the newly encountered base station 120. If the relative pathdelay is positive, then mobile station 110 may determine that the basestation 120 is farther away from base station 130. Accordingly, mobilestation 110 may not hand-over the signal to the newly encounteredsignals from base station 120. However, if the relative path delay isnegative, then mobile station 110 may determine that the base station120 is closer than base station 130 and may hand-over the signal to thenewly encountered signals from base station 120.

It should be understood, that although the relative path delay wasdescribed above in the context of hand-over, the relative path delaybetween two or more base stations to a mobile station can also be usedfor fast-base station switching (FBSS) or macro-diversity handover(MDHO).

Each base station 120 and 130 includes respective transmittercircuitries 126 and 136. Each base station 120 and 130 also includes arespective memory 124 and 134. Transmitter circuitries 126 and 136transmit the data signals with a respective preamble sequence thatidentifies the transmitter (and thereby the base station) from which thedata signal originate. In particular, transmitter circuitry 126 and 136may transmit data signals in accordance with the OFDM standard. Memories124 and 134 store preamble sequences that may be needed to generate thetransmitted signals.

The preamble sequence used to identify each base station 120 and 130 maybe a preamble OFDM symbol unique to the base station. For example, asdiscussed in more detail below, each base station transmits data in aset number of frames and each frame is prefaced with a preamble symbolunique to the transmitter. The mobile station (i.e., the receivingdevice) can identify a base station based on the unique preamble symbolassociated with that base station.

Base stations 120 and 130 may communicate with each other in order tosynchronize the transmitted signals. In particular, each base stationmay be synchronized such that the signals are transmitted substantiallyat the same time. If the signals were not synchronized, the relativepath delay computation may lead to inaccurate results. However, in someimplementations, if the transmitted signals are not synchronized, thesignals may be time-stamped to enable the mobile station 110 tocompensate for the transmission delay added by lack of synchronization.Compensating for such transmission delay may enable the mobile stationto compute the relative path delay (and thereby the distance to eachbase station) even though the signals are not synchronized.

Mobile station 110 may include a memory that stores at least one OFDMsymbol whose frequency domain values are known to mobile station 110.This symbol may be referred to below as the transmitted preamble OFDMsymbol. In particular, mobile station 110 may be configured to know whatsignal should be transmitted from a base station based on a predefinedstandard (i.e., WiMax). Mobile station 110 may compute the path delay toa base station based on a comparison between the expected signal (i.e.,the transmitted signal X(t)) and the actually received signal Y(t).

Each transmitter 126 and 136 transmits a signal X_(i)[k] correspondingto a preamble sequence i at subcarrier k. In WiMax, the transmittedsignal X_(i)[k] has values of +1 and −1 at the preamble subcarrier setcorresponding to a given segment and has values of 0 at othersubcarriers. In particular, a preamble sequence in the WiMax standardtypically includes three subcarrier sets such that three differentindices of k are selected for transmission in the preamble sequence.

For example, as discussed above, each transmitter 126 and 136 inserts apreamble OFDM symbol at the start of each frame of data. To generate thepreamble OFDM symbol, the transmitters select one of 114 preamblesequences (i.e., in the case of a 1024 or 512 FFT mode) based on theparticular IDCell number of the base station and the segment number. Forexample, memories 124 and 134 may store a table for providing theparticular preamble sequence to generate. The table may include columnsfor the index k, the IDcell number of the base station, the segmentnumber (for each of the three transmitted segments in WiMax) and thesequence of hexadecimal bits for the preamble sequence. The transmittermay address a particular row in the table (based on the index, IDcelland segment number) to retrieve the appropriate preamble sequence fortransmission.

The subcarriers are modulated (e.g., using phase shift keying (PSK)modulation) with the selected preamble sequence. In someimplementations, only one out of every three subcarriers is modulated.For example, the preamble subcarrier set for segment n may be computedin accordance with PreambleCarrierSet_(n)=n+3*k where k is an integerbetween 0 and 283 for a 10.24 FFT and is an integer between 0 and 142for 512 FFT. 86 guard subcarriers may be inserted on the left and rightsides of the spectrum for a 1024 FFT and 42 guard subcarriers may beinserted on the left side and 41 on the right for a 512 FFT. In someimplementations, the power may be increased by, for example, eighttimes. Preamble modulation techniques are described in more detail in H.Arslan, Signal Processing Communications Handbook, CRC Press, 2004 andT. Yucek and H. Arsland, Delay spread and time dispersion estimation foradaptive OFDM systems, Proc. IEEE WCNC, pp. 1433-1438, April 2006, eachof which is hereby incorporated by reference herein in its entirety.

The modulated subcarriers are converted to a time-domain signal using,for example, an Inverse Fast Fourier Transform for transmission.Finally, a cyclic prefix is added to the time-domain signal.

The received preamble symbol Y[k] of the k-th subcarrier received bymobile station 110 may be represented in the frequency domain asfollows:

$\begin{matrix}{{Y_{1}\lbrack k\rbrack} = {{\sum\limits_{i = 0}^{l - 1}\;{{H_{1,i}\lbrack k\rbrack}{X_{i}\lbrack k\rbrack}}} + {Z_{i}\lbrack k\rbrack}}} \\\vdots \\{{Y_{R}\lbrack k\rbrack} = {{\sum\limits_{i = 0}^{l - 1}\;{{H_{R,i}\lbrack k\rbrack}{X_{i}\lbrack k\rbrack}}} + {Z_{R}\lbrack k\rbrack}}}\end{matrix}$where k is the subcarrier index, i is the preamble sequence number whichis the base station index number that represents the unique identifierof the base station, I is the total number of distinct preamblesequences (i.e., 114 for 1024 FFT), R is the number of receiver antennasin the mobile station (e.g., if more than one antenna is used in a MIMOimplementation), Y_(r)[k] and Z_(r)[k] represent the received signal andnoise (plus interference), respectively, of the antenna r at thesubcarrier k, and H_(r,i)[k] represents the channel gain from the basestation with the i-th preamble sequence to antenna r at subcarrier k. Asdiscussed above, X_(i)[k] is the transmitted signal corresponding to thepreamble sequence i associated with a particular base station atsubcarrier k. Since mobile station 110 knows the transmitted values ofX_(i)[k], mobile station 110 may compare the expected transmitted valuesX_(i)[k] with received values Y_(R)[k] to compute the relative pathdelay estimation as discussed in more detail below in connection withFIGS. 2-4.

The preamble sequence corresponding to i=0 may denote the serving basestation while all other i values denote base station signals acquired bythe mobile station from other base stations.

Mobile station 110 may compute the relative path delay estimation basedon the signals received from two or more base stations 120 and 130. Inparticular, mobile station 110 may communicate with a serving basestation and when mobile station 110 acquires a signal from a new basestation, mobile station 110 may compare the path delays to select thesignal from the closest base station. Mobile station 110 may compute therelative path delay based on either a channel estimation algorithm or adifferential algorithm. The algorithms for computing the relative pathdelay are discussed in more detail in connection with FIGS. 3 and 4.

FIG. 2 is a diagram of an illustrative relative path delay estimationcircuit 200 in accordance with an embodiment of the present invention.Relative path delay estimation circuit 200 may be implemented in mobilestation 110. Relative path delay estimation circuit 200 may include acontroller 230, a receiver/processing circuit 240, a memory 220 and apath delay estimation circuit 210. Although each component isillustrated in FIG. 2 separately, it should be understood that thecomponents may be combined into one or more different devices orcomponents.

Controller 230 may receive an indication in the form of an interruptfrom, for example, receiver/processing circuit 240 indicating that anew. base station signal has been detected or acquired. For example,mobile station 110 may be communicating with base station 120. Upondetecting or acquiring a signal from another base station 130 (e.g.,because mobile station 110 moves around and may be in range of receivingsignals from another base station), receiver 240 may instruct controller230 to conduct a relative path delay analysis. This may be done todetermine which base station 120 or 130 is closer to mobile station 110in order to select the signals from the closer base station.

Controller 230 may instruct path delay estimation circuit 210 to performan algorithm to determine which base station is closer. Path delayestimation circuit 210 may perform a channel estimation algorithm or adifferential algorithm to determine which base station is closer.Receiver/processing circuit 240 may compute a Fast Fourier Transform(FFT) and various other operations to retrieve the preamble sequencefrom the received signal 122. Receiver/processing circuit 240 may alsocompute and provide the conjugate (denoted by ‘*’) of the preamblesequence. The preamble sequence and its conjugate may be stored inmemory 220.

Path delay estimation circuit 210 may receive the received signalpreamble sequence (S1) (and its conjugate if necessary) corresponding tothe newly acquired signals 122 from the base station throughcommunications link 242. Alternatively, path delay estimation circuit210 may retrieve the preamble sequence corresponding to signals 122 frommemory 220. Path delay estimation circuit 210 may also retrieve thepreamble sequence (S2) corresponding to the serving base station signals132 from memory 220 through communications link 222.

A table that includes predefined preamble sequences (e.g., by the WiMaxstandard) for various base stations and segment identifiers may also bestored in memory 220. Storing this information in memory 220 may enablemobile station 110 to know what are the expected transmitted preamblesequences (or what the transmitted preamble sequences should be) foreach received signal. Path delay estimation circuit 210 may retrievethis information from memory 220 by providing the appropriate tableindices (or address signals) 212.

Path delay estimation circuitry 210 may compute the relative path delay214 between the two base stations based on the received and transmittedsignals and by applying either the channel estimate or differentialalgorithm. For example, path delay estimation circuitry 210 may computethe difference between the two distances of each base station 120 and130 (determined based on the phase angles) and determine which basestation is closer to mobile station 110. Path delay estimation circuitry210 provides the relative path delay 214 to controller 230. Controller230 may switch from one serving base station 130 to another base station120 if it determines that the newly acquired signals from base station120 are closer to mobile station 110 than the signals from serving basestation 130.

Relative path delay estimation circuit 200 may also include a servingsource selector circuit 250 and a utilization circuit 260. Servingsource selector circuit 250 receives the computed path delay 214 andselects the base station closest to mobile station 110 to be the servingbase station. For example, serving source selector circuit 250 mayreceive the signals 122/132 from multiple base stations. Based on thedetermination of the relative path delay between the base stations,serving source selector circuit 250 selects one of the signals 122/132to provide to a utilization circuit 260 via communications link 252.Utilization circuit 260 may retrieve the data from the selected signaland perform a variety of computations on the data. For example,utilization circuit 260 may compute an FFT, IFFT, various signalprocessing operations, voice retrieval computations, or the like.

FIG. 3 is a detailed diagram of a channel estimation based relative pathdelay estimation circuit 300 in accordance with an embodiment of thepresent invention. In some embodiments, relative path delay estimationcircuit 210 computes the path delay between two or more signals usingchannel estimation based relative path delay estimation circuit 300.

Channel estimation based relative path delay estimation circuit 300 mayinclude a channel estimation circuit 310, filter circuitry 320, delaymetric estimation circuit 330 and relative path delay circuitry 340.Channel estimation circuit 310 receives the preamble sequence 242Y_(r)[k] corresponding to a base station received signal from receiver240 (FIG. 2). Channel estimation circuit 310 also receives from memory220 (FIG. 2) the expected (or transmitted) preamble sequence 212X_(j)*[k] (in conjugate form) that corresponds to the base station j.Alternatively, channel estimation circuit 310 may receive the expected(or transmitted) preamble sequence 212 in regular form and compute theconjugate.

Channel estimation circuit 310 computes the initial channel estimate forthe preamble sequence j associated with a signal received from basestation 120 or 130. For example, channel estimation circuit 310 maycompute the initial channel estimate in accordance with the following:H _(r,j,init) [k]=Y _(r) [k]X _(j) *[k] for kεP _(j)where P_(j) is the set of the indices of subcarriers used for thepreamble sequence associated with the base station and r identifies aparticular one of R antennas. For example, in WiMax implementations, theP_(j) set would include three different values. Accordingly, threedifferent initial channel estimates may be computed, one for eachpreamble sequence subcarrier index.

Channel estimation circuit 310 may provide the computed initial channelestimate H to filter circuitry 320. Filter circuitry 320 may be used tocombine all of the computed initial channel estimates associated witheach preamble sequence subcarrier index. For example, in WiMaximplementations, filter circuitry 320 may accumulate the three initialchannel estimates that are computed.

Filter circuitry 320 may use a variety of computations to combine theinitial channel estimates received from channel estimation circuitry310. For example, filter circuitry 320 may implement any linear filter,where the linear minimum mean square error is used. Alternatively,filter circuitry 320 may implement a 3-tap (or any size tap) localaverage filter or maximum likelihood channel estimation algorithm tocompute the final channel estimate.

In general terms, filter circuitry 320 may compute the final channelestimate based on the initial channel estimate in accordance with thefollowing:

${{\hat{H}}_{r,j}\lbrack m\rbrack} = {\sum\limits_{k \in P_{j}}\;{a_{m,k}{H_{r,j,{init}}\lbrack k\rbrack}}}$where Ĥ_(r,j)[m] is the final channel estimate at the subcarrier m anda_(m,k) is a filter coefficient.

The final channel estimate is provided to delay metric estimate circuit330 for computing the relative path delay. For example, delay metricestimate circuit 330 may correlate the final channel estimate at thereceived subcarrier values k with the final channel estimate a shiftedsubcarrier values (k−q). The correlation may be performed andaccumulated for each receiving antenna 112 (represented by r). For WiMaximplementations, the final channel estimate is correlated with a finalchannel estimate at a subcarrier value shifted by three.

In general terms, delay metric estimate circuit 330 may compute the pathdelay associated with base station 130 (represented by j) in accordancewith the following:

${\hat{\theta}}_{j,{chan}} = {{- \frac{N}{2\pi\; q}}{\angle\left( {\sum\limits_{r = 1}^{R}\;{\sum\limits_{m \in {\overset{\sim}{P}}_{j}}\;{{{\hat{H}}_{r,j}^{*}\lbrack m\rbrack}{{\hat{H}}_{r,j}\left\lbrack {m + q} \right\rbrack}}}} \right)}}$where {circumflex over (θ)}_(j,chan) is the path delay corresponding tobase station j, R is the number of receiver antennas (this may beunnecessary if only one antenna 112 is presented (e.g., in non MIMObased systems), {tilde over (P)}_(j) is the set of indices ofsubcarriers used for preamble sequence j exclusing the highestsubcarrier index and Ĥ_(r,j) is the final channel estimate provided byfilter circuitry 320 or in some implementations directly from channelestimation circuitry 310 (e.g., when no filter is necessary).

The relative path delay is computed based on the path delay computed bydelay metric estimate circuit 330. Relative path delay computationcircuit 340 receives the computed path delay associated with aparticular base station (e.g., base station j or base station 130) fromdelay metric estimate circuit 330 and compares it with a path delayassociated with another base station (e.g., a serving base station j=0or base station 130). For example, relative path delay computationcircuit 340 may compute the difference between the two path delays todetermine if the relative path delay is positive or negative. A positivevalue may indicate that base station j or base station 120 is closer tomobile station 110 than the serving base station 130. Conversely, anegative value may indicate that base station j or base station 120 isfurther away from mobile station 110 than the serving base station 130.It should be understood that alternatively, a negative value may be usedto indicate a base station is closer while a positive value may be usedto indicate a base station is farther away.

In general terms, relative path delay computation circuit 340 maycompute the relative path delay between two or more base stations inaccordance with the following:Δ_(j)={circumflex over (θ)}_(j,chan)−{circumflex over (θ)}_(0,chan)where j represents a newly acquired signal from a base station otherthan the serving base station and {circumflex over (θ)}_(0,chan)represents the path delay associated with the serving base station. Fromthe above equation, it can be seen that when the path delay associatedwith base station 120 corresponding to j is larger than the path delayassociated with the serving base station, the relative phase delay ispositive indicating that the serving base station is closer. Conversely,when the path delay associated with base station 130 corresponding to jis smaller than the path delay associated with the serving base station,the relative phase delay is negative indicating that the serving basestation is farther away from mobile station 110.

FIG. 4 is a detailed diagram of a differential based relative path delayestimation circuit 400 in accordance with an embodiment of the presentinvention. In some embodiments, relative path delay estimation circuit210 computes the path delay between two or more signals usingdifferential based relative path delay estimation circuit 400.

Differential based relative path delay estimation circuit 400 mayinclude a differential computation circuit 410, a delay metricestimation circuit 420 and a relative path delay circuit 430.Differential computation circuit 410 receives preamble sequence 242Y_(r)[k] corresponding to a base station and correlates the receivedpreamble sequence with a conjugate preamble sequence evaluated at ashifted preamble sequence index value. For example, the conjugate may bethe received preamble sequence index shifted over by a value q (e.g., qis equal to three for WiMax implementations). In general, differentialcomputation circuitry 410 may compute the differential of the receivedpreamble sequence in accordance with the following:M _(r) [k]=Y _(r) *[k]Y _(r) [k+q]where M_(r)[k] is the differential of the received signal for the k-thpreamble sequence index value, r is the receiver antenna (e.g., for MIMOimplementations having signals received over multiple antennas) and q isthe index value of the preamble sequence with which the receivedpreamble sequence is correlated.

Differential computation circuitry 410 may receive the conjugate of thereceived preamble sequence Y[k] or differential computation circuitry410 may compute the conjugate based on the received preamble sequence242 Y[k].

Differential computation circuitry 410 also computes the differential ofthe transmitted signal in a similar manner as computing the differentialof the received signal. Differential computation circuitry 410 receivesfrom memory 220 (FIG. 2), the preamble sequence of the transmittedsignal 212. In particular, differential computation circuitry 410determines based on the value of j which preamble sequence is expectedby retrieving the corresponding preamble sequence from memory 212. Sincethe standard is defined (e.g., based on WiMax standard), the preamblesequence values at different index values are known and can be comparedwith the received preamble sequence values.

Differential computation circuitry 410 retrieves from the memory thetransmitted preamble sequence value from the memory and a conjugatepreamble sequence evaluated at a shifted preamble sequence index value.Alternatively, differential computation circuitry 410 may compute theconjugate based on the transmitted preamble sequence value 212 receivedfrom the memory. Differential computation circuitry 410 correlates thetransmitted preamble sequence value with the conjugate preamble sequenceevaluated at a shifted preamble sequence index value.

In general, differential computation circuitry 410 may compute thedifferential of the transmitted preamble sequence in accordance with thefollowing:D _(j) [k]=X _(j) [k+q]X _(j) *[k]where D_(j)[k] is the differential of the transmitted signal for thek-th preamble sequence index value, j is the preamble sequenceassociated with the base station which is known to transmit the receivedpreamble sequence and q is the index value of the preamble sequence withwhich the transmitted preamble sequence is correlated.

Preferably, the preamble sequences that are correlated for the receivedand the transmitted preamble sequences are shifted by the same amount.In particular, when the differential is computed for the receivedpreamble sequence, a correlation is made between the received preamblesequence and the conjugate preamble sequence evaluated at a shiftedpreamble sequence value q (e.g., q=3). Similarly, when the differentialis computed for the transmitted preamble sequence, a correlation is madebetween the transmitted preamble sequence and the transmitted conjugatepreamble sequence evaluated at a preamble sequence value shifted by thesame amount q (i.e., q=3).

Delay metric estimation circuitry 420 computes the path delaycorresponding to the base station j. Delay metric estimation circuitry420 computes the path delay based on the differential received preamblesequence and the differential transmitted preamble sequence. Delaymetric estimation circuitry 420 may compute the conjugate of thedifferential of the transmitted preamble sequence or may receive aconjugate of the transmitted preamble sequence computed by differentialcomputation circuitry 410. Delay metric estimation circuitry 420correlates the differential of the received preamble sequence with aconjugate of the differential of the transmitted preamble sequence forevery antenna r and all of the set of the indices P_(j) of subcarriersused for the preamble sequence j associated with the base station.

In general delay metric estimation circuitry 420 may compute the pathdelay corresponding to the base station j in accordance with thefollowing:

${\hat{\theta}}_{j,{diff}} = {{- \frac{N}{2\pi\; q}}{\angle\left( {\sum\limits_{r = 1}^{R}\;{\sum\limits_{k \in {\overset{\sim}{P}}_{j}}^{\;}\;{{M_{r}\lbrack k\rbrack}{D_{j}^{*}\lbrack k\rbrack}}}} \right)}}$where {circumflex over (θ)}_(j,diff) is the path delay corresponding tobase station j.

The relative path delay is computed based on the path delay computed bydelay metric estimation circuit 420. Relative path delay circuit 430computes the relative path delay in a similar manner as relative pathdelay circuit 340. Relative path delay computation circuit 430 receivesthe computed path delay associated with a particular base station (e.g.,base station j or base station 120) from delay metric estimate circuit420 and compares it with a path delay associated with another basestation (e.g., a serving base station j=0 or base station 130). Forexample, relative path delay computation circuit 430 may compute thedifference between the two path delays to determine if the relative pathdelay is positive or negative. A positive value may indicate that basestation j or base station 120 is closer to mobile station 110 than theserving base station 130. Conversely, a negative value may indicate thatbase station j or base station 120 is further away from mobile station110 than the serving base station 130. It should be understood thatalternatively, a negative value may be used to indicate a base stationis closer while a positive value may be used to indicate a base stationis farther away.

In general terms, relative path delay computation circuit 430 maycompute the relative path delay between two or more base stations inaccordance with the following:

Δ_(j)={circumflex over (θ)}_(j,chan)−{circumflex over (θ)}_(0,chan)

where j represents a newly acquired signal from a base station 120 otherthan the serving base station 130 and {circumflex over (θ)}_(0,chan)represents the path delay associated with the serving base station 130.

Computing the relative path delay using the differential algorithm isless complex than computing the relative path delay using channelestimation algorithm. In particular, the relative path delay may becomputed using the differential algorithm without computing the channelestimate. Additionally, in the differential algorithm, the magnitude ofthe correlation between the received preamble sequence correlation andthe transmitted preamble sequence correlation reflects the RSSI of thereceived signal. Thus, it may be desirable to share the output of delaymetric estimation circuitry 420 with another system component that needsthe value of the RSSI of the received signal. In such a scenario, itshould be understood that the output may be an intermediate step incomputing the path delay and not necessarily the overall output providedto relative path delay circuit 430. In particular, delay metricestimation circuitry 420 may compute M[k]D*[k] and accumulate the valuefor various components (e.g., the values at different antennas and indexvalues for k). However, the RSSI may be reflected without theaccumulation and simply by computing the magnitude of M[k]D*[k]. Thecomputed magnitude may be provided to other system components that needthe RSSI value without interrupting the computation of the path delayassociated with the base station.

In some implementations, the instantaneous values of the path delaysassociated with the base station j (whether computed using the channelestimation or differential algorithm) may be averaged (e.g., using alow-pass filter) over multiple frames. This may provide a more accuratevalue for the path delay. Accordingly, in such implementations, therelative path delay circuitries 340 and 430 may receive the averagedpath delays instead of the instantaneous path delays computed by delaymetric estimation circuitries 330 and 420. Delay metric estimationcircuitries 330 and 420 may store in a memory the instantaneous valuesof the path delays associated with different frames of a signal receivedfrom a base station and average the stored values after a particularnumber of frames are received.

FIG. 5 illustrates a process 500 for computing relative path delaybetween multiple sources in accordance with an embodiment of the presentinvention. At step 510, a first signal transmitted from a first sourceand a second signal transmitted from a second source are received at adevice. For example, mobile station 110 receives a first signal (e.g., afirst preamble sequence) from base station 130 and a second signal(e.g., a second preamble sequence) from base station 120 (FIG. 1). Basestation 130 may be the serving base station while the signal from basestation 120 may be acquired at a later time (or substantially the sametime).

At step 520, a determination is made as to which one of the first sourceand second source is closer to the device based on a relative path delaymeasurement. For example, mobile station 110 may compute the path delayassociated with each base station. In some embodiments, mobile stationmay compute the path delay associated with the first base station andstore it to a memory. When a signal from a second base station isreceived, the path delay associated with the second base station iscomputed and compared with the stored path delay associated with thefirst base station. Mobile station 110 may compute the path delay usinga channel estimation based algorithm (FIG. 2) or a differential basedalgorithm (FIG. 3). The path delays associated with each signal arecombined (e.g., by computing the difference between the path delays) andbased on the combination, mobile station 110 determines which one of thebase stations 120 and 130 is closer to mobile station 110.

At step 530, the signal from the first or the second source is selectedbased on the determination of which one of the first source and secondsource is closer to the device. For example, mobile device 110 mayswitch from one serving base station to another if it determines (basedon the channel estimation algorithm or differential algorithm) that onebase station is closer to mobile station 110 than the other.

Referring now to FIGS. 6A-6C, various exemplary implementations of thepresent invention are shown.

Referring now to FIG. 6A, the present invention implements a controlsystem of a vehicle 630, a WLAN interface and/or mass data storage ofthe vehicle control system. In some implementations, the presentinvention may implement a powertrain control system 634 that receivesinputs from one or more sensors such as temperature sensors, pressuresensors, rotational sensors, airflow sensors and/or any other suitablesensors and/or that generates one or more output control signals such asengine operating parameters, transmission operating parameters, and/orother control signals.

The present invention may also be implemented in other control systems639 of the vehicle 630. The control system 639 may likewise receivesignals from input sensors 637 and/or output control signals to one ormore output devices 638. In some implementations, the control system 639may be part of an anti-lock braking system (ABS), a navigation system, atelematics system, a vehicle telematics system, a lane departure system,an adaptive cruise control system, a vehicle entertainment system suchas a stereo, DVD, compact disc and the like. Still other implementationsare contemplated.

The powertrain control system 634 may communicate with mass data storage631 that stores data in a nonvolatile manner. The mass data storage 631may include optical and/or magnetic storage devices for example harddisk drives HDD and/or DVDs. The HDD may be a mini HDD that includes oneor more platters having a diameter that is smaller than approximately1.8″. The powertrain control system 634 may be connected to memory 632such as RAM, ROM, low latency nonvolatile memory such as flash memoryand/or other suitable electronic data storage. The powertrain controlsystem 634 also may support connections with a WLAN via a WLAN networkinterface 633. The control system 639 may also include mass datastorage, memory and/or a WLAN interface (all not shown).

Referring now to FIG. 6B, the present invention can be implemented in acellular phone 650 that may include a cellular antenna 651. The presentinvention may implement either or both signal processing and/or controlcircuits, which are generally identified in FIG. 6B at 652, a WLANinterface and/or mass data storage of the cellular phone 650. In someimplementations, the cellular phone 650 includes a microphone 656, anaudio output 658 such as a speaker and/or audio output jack, a display660 and/or an input device 662 such as a keypad, pointing device, voiceactuation and/or other input device. The signal processing and/orcontrol circuits 652 and/or other circuits (not shown) in the cellularphone 650 may process data, perform coding and/or encryption, performcalculations, format data and/or perform other cellular phone functions.

The cellular phone 650 may communicate with mass data storage 664 thatstores data in a nonvolatile manner such as optical and/or magneticstorage devices for example hard disk drives HDD and/or DVDs. The HDDmay be a mini HDD that includes one or more platters having a diameterthat is smaller than approximately 1.8″. The cellular phone 650 may beconnected to memory 666 such as RAM, ROM, low latency nonvolatile memorysuch as flash memory and/or other suitable electronic data storage. Thecellular phone 650 also may support connections with a WLAN via a WLANnetwork interface 668.

Referring now to FIG. 6C, the present invention can be implemented in amedia player 670. The present invention may implement either or bothsignal processing and/or control circuits, which are generallyidentified in FIG. 6C at 674, a WLAN interface and/or mass data storageof the media player 670. In some implementations, the media player 670includes a display 676 and/or a user input 677 such as a keypad,touchpad and the like. In some implementations, the media player 670 mayemploy a graphical user interface (GUI) that typically employs menus,drop down menus, icons and/or a point-and-click interface via thedisplay 676 and/or user input 677. The media player 670 further includesan audio output 675 such as a speaker and/or audio output jack. Thesignal processing and/or control circuits 674 and/or other circuits (notshown) of the media player 670 may process data, perform coding and/orencryption, perform calculations, format data and/or perform any othermedia player function.

The media player 670 may communicate with mass data storage 671 thatstores data such as compressed audio and/or video content in anonvolatile manner. In some implementations, the compressed audio filesinclude files that are compliant with MP3 format or other suitablecompressed audio and/or video formats. The mass data storage 671 mayinclude optical and/or magnetic storage devices for example hard diskdrives HDD and/or DVDs. The HDD may be a mini HDD that includes one ormore platters having a diameter that is smaller than approximately 1.8″.The media player 670 may be connected to memory 672 such as RAM, ROM,low latency nonvolatile memory such as flash memory and/or othersuitable electronic data storage. The media player 670 also may supportconnections with a WLAN via a WLAN network interface 673. Still otherimplementations in addition to those described above are contemplated.

The foregoing describes systems and methods for measuring relative pathdelay in systems having multiple transmitting devices. The abovedescribed embodiments of the present invention are presented for thepurposes of illustration and not of limitation. Furthermore, the presentinvention is not limited to a particular implementation. The inventionmay be implemented in hardware, such as on an application specificintegrated circuit (ASIC) or on a field-programmable gate array (FPGA).The invention may also be implemented in software.

1. A method comprising: receiving, at a device, a first signaltransmitted from a first source and a second signal transmitted from asecond source; determining which one of the first source and the secondsource is closer to the device based on a relative path delaymeasurement, wherein the relative path delay measurement is computedbased on at least one of a channel estimate of the first source and adifferential of the first signal transmitted from the first source; andselecting the corresponding signal from the first or the second sourcebased on the determination of which one of the first source and thesecond source is closer to the device.
 2. The method of claim 1 whereinthe respective signals transmitted from the first and second sources aresynchronized.
 3. The method of claim 1 wherein the relative path delaymeasurement is derived by: measuring a path delay of the first sourcerelative to the second source, such that the source with the smallerpath delay is determined to be the source closer to the device.
 4. Themethod of claim 1 wherein the first and second sources are base stationsand the device is a mobile station.
 5. The method of claim 4 wherein thebase stations are mobile phone antennas or wireless access points andthe device is a mobile phone or computing device.
 6. The method of claim1 wherein each of the first source and the second source has a distinctpreamble Orthogonal Frequency Division Multiplexing (OFDM) symbol. 7.The method of claim 1 wherein determining which one the first source andthe second source is closer to the device comprises: computing a channelestimate that corresponds to the first source using a first preamblesequence associated with the first source; and measuring a first phaseangle that corresponds to the first source based on the channelestimate.
 8. The method of claim 7 further comprising: measuring asecond phase angle that corresponds to the second source based on achannel estimate that corresponds to a second preamble sequenceassociated with the second source; and computing a difference betweenthe first and the second phase angles to determine which one of thefirst source and the second source is closer to the device.
 9. Themethod of claim 7 wherein the channel estimate is computed in accordancewith a maximum likelihood (ML) algorithm.
 10. The method of claim 7further comprising filtering the channel estimate.
 11. The method ofclaim 10 wherein the channel estimate is filtered using a low passfilter, linear minimum mean square error (LMMSE), or local averagingestimation.
 12. The method of claim 7 wherein measuring a first phaseangle comprises computing a correlation between the channel estimate ata first subcarrier index and the channel estimate at a differentsubcarrier index.
 13. The method of claim 12 wherein the differentsubcarrier index is the first subcarrier index shifted by a value equalto a subcarrier interval of the first preamble sequence.
 14. The methodof claim 1 wherein determining which one of the first source and thesecond source is closer to the device comprises: computing adifferential of the first received signal; computing a differential ofthe first transmitted signal for a first preamble sequence associatedwith the first source; and estimating a first phase angle thatcorresponds to the first source based on the differential of the firstreceived signal and the differential of the first transmitted signal.15. The method of claim 14 further comprising: estimating a second phaseangle that corresponds to the second source based on a differential thatcorresponds to a second preamble sequence associated with the secondsource; and computing a difference between the first and the secondphase angles to determine which one of the first source and the secondsource is closer to the device.
 16. The method of claim 14 whereincomputing the differential of the first received signal comprisescorrelating the first received signal at a first subcarrier index withthe first received signal at the first subcarrier index shifted by asubcarrier interval of the first preamble sequence.
 17. The method ofclaim 14 wherein computing the differential of the first transmittedsignal comprises correlating the first transmitted signal at a firstsubcarrier index with the first transmitted signal at the firstsubcarrier index shifted by a subcarrier interval of the first preamblesequence.
 18. The method of claim 1 wherein selecting the correspondingsignal from the first or the second source comprises switching from thefirst source to the second source.
 19. A device comprising: a receiverthat receives a first signal transmitted from a first source and asecond signal transmitted from a second source; control circuitry that:determines which one of the first source and the second source is closerto the device based on a relative path delay measurement, wherein therelative path delay measurement is computed based on at least one of achannel estimate of the first source and a differential of the firstsignal transmitted from the first source; and selects the correspondingsignal from the first or the second source based on the determination ofwhich one of the first source and the second source is closer to thedevice.
 20. The device of claim 19 wherein the respective signalstransmitted from the first and second sources are synchronized.
 21. Thedevice of claim 19 wherein the control circuitry: measures a path delayof the first source relative to the second source, such that the sourcewith the smaller path delay is determined to be the source closer to thedevice.
 22. The device of claim 19 wherein the first and second sourcesare base stations and the device is a mobile station.
 23. The device ofclaim 22 wherein the base stations are mobile phone antennas or wirelessaccess points and the device is a mobile phone or computing device. 24.The device of claim 19 wherein each of the first source and the secondsource has a distinct preamble Orthogonal Frequency DivisionMultiplexing (OFDM) symbol.
 25. The device of claim 19 wherein thecontrol circuitry that determines which one of the first source and thesecond source is closer: computes a channel estimate that corresponds tothe first source using a first preamble sequence associated with thefirst source; and measures a first phase angle that corresponds to thefirst source based on the channel estimate.
 26. The device of claim 25wherein the control circuitry: measures a second phase angle thatcorresponds to the second source based on a channel estimate thatcorresponds to a second preamble sequence associated with the secondsource; and computes a difference between the first and the second phaseangles to determine which one of the first source and the second sourceis closer to the device.
 27. The device of claim 25 wherein the channelestimate is computed in accordance with a maximum likelihood (ML)algorithm.
 28. The device of claim 25 wherein the control circuitryfilters the channel estimate.
 29. The device of claim 28 wherein thechannel estimate is filtered using a low pass filter, linear minimummean square error (LMMSE), or local averaging estimation.
 30. The deviceof claim 25 wherein the control circuitry that measures a first phaseangle computes a correlation between the channel estimate at a firstsubcarrier index and the channel estimate at a different subcarrierindex.
 31. The device of claim 30 wherein the different subcarrier indexis the first subcarrier index shifted by a value equal to a subcarrierinterval of the first preamble sequence.
 32. The device of claim 19wherein the control circuitry that determines which one of the firstsource and the second source is closer to the device: computes adifferential of the first received signal; computes a differential ofthe first transmitted signal for a first preamble sequence associatedwith the first source; and estimates a first phase angle thatcorresponds to the first source based on the differential of the firstreceived signal and the differential of the first transmitted signal.33. The device of claim 32 wherein the control circuitry: estimates asecond phase angle that corresponds to the second source based on adifferential that corresponds to a second preamble sequence associatedwith the second source; and computes a difference between the first andthe second phase angles to determine which one of the first source andthe second source is closer to the device.
 34. The device of claim 32wherein the control circuitry that computes the differential of thefirst received signal correlates the first received signal at a firstsubcarrier index with the first received signal at the first subcarrierindex shifted by a subcarrier interval of the first preamble sequence.35. The device of claim 32 wherein the control circuitry that computesthe differential of the first transmitted signal correlates the firsttransmitted signal at a first subcarrier index with the firsttransmitted signal at the first subcarrier index shifted by a subcarrierinterval of the first preamble sequence.
 36. The device of claim 19wherein the control circuitry that selects the corresponding signal fromthe first or the second source switches from the first source to thesecond source.
 37. A device comprising: means for receiving a firstsignal transmitted from a first source means and a second signaltransmitted from a second source means; means for determining which oneof the first source and the second source means is closer to the devicemeans based on a relative path delay measurement, wherein the relativepath delay measurement is computed based on at least one of a channelestimate of the first source means and a differential of the firstsignal transmitted from the first source means; and means for selectingthe corresponding signal from the first or the second source means basedon the determination of which one of the first source and the secondsource means is closer to the device means.
 38. The device of claim 37wherein the respective signals transmitted from the first and secondsource means are synchronized.
 39. The device of claim 37 wherein therelative path delay measurement is derived by: means for measuring apath delay of the first source means relative to the second sourcemeans, such that the source means with the smaller path delay isdetermined to be the source closer to the device means.
 40. The deviceof claim 37 wherein the first and second source means are base stationsand the device means is a mobile station.
 41. The device of claim 40wherein the base stations are mobile phone antennas or wireless accesspoints and the device is a mobile phone or computing device.
 42. Thedevice of claim 37 wherein each of the first source and the secondsource means has a distinct preamble Orthogonal Frequency DivisionMultiplexing (OFDM) symbol.
 43. The device of claim 37 wherein the meansfor determining which one of the first source and the second sourcemeans is closer to the device means comprises: means for computing achannel estimate that corresponds to the first source means using afirst preamble sequence associated with the first source means; andmeans for measuring a first phase angle that corresponds to the firstsource means based on the channel estimate.
 44. The device of claim 43further comprising: means for measuring a second phase angle thatcorresponds to the second source means based on a channel estimate thatcorresponds to a second preamble sequence associated with the secondsource means; and means for computing a difference between the first andthe second phase angles to determine which one of the first source andthe second source means is closer to the device means.
 45. The device ofclaim 43 wherein the channel estimate is computed in accordance with amaximum likelihood (ML) algorithm.
 46. The device of claim 43 furthercomprising filtering the channel estimate.
 47. The device of claim 46wherein the channel estimate is filtered using a low pass filter, linearminimum mean square error (LMMSE), or local averaging estimation. 48.The device of claim 43 wherein means for measuring a first phase anglecomprises means for computing a correlation between the channel estimateat a first subcarrier index and the channel estimate at a differentsubcarrier index.
 49. The device of claim 48 wherein the differentsubcarrier index is the first subcarrier index shifted by a value equalto a subcarrier interval of the first preamble sequence.
 50. The deviceof claim 37 wherein means for determining which one of the first sourceand the second source means is closer to the device means comprises:means for computing a differential of the first received signal; meansfor computing a differential of the first transmitted signal for a firstpreamble sequence associated with the first source means; and means forestimating a first phase angle that corresponds to the first sourcemeans based on the differential of the first received signal and thedifferential of the first transmitted signal.
 51. The device of claim 50further comprising: means for estimating a second phase angle thatcorresponds to the second source means based on a differential thatcorresponds to a second preamble sequence associated with the secondsource means; and means for computing a difference between the first andthe second phase angles to determine which one of the first source andthe second source means is closer to the device means.
 52. The device ofclaim 50 wherein means for computing the differential of the firstreceived signal comprises means for correlating the first receivedsignal at a first subcarrier index with the first received signal at thefirst subcarrier index shifted by a subcarrier interval of the firstpreamble sequence.
 53. The device of claim 50 wherein means forcomputing the differential of the first transmitted signal comprisesmeans for correlating the first transmitted signal at a first subcarrierindex with the first transmitted signal at the first subcarrier indexshifted by a subcarrier interval of the first preamble sequence.
 54. Thedevice of claim 37 wherein means for selecting the corresponding signalfrom the first or the second source means comprises switching from thefirst source means to the second source means.